1. Technical Field
This invention relates generally to rocket thrusters and more particularly, to small regeneratively cooled rocket thruster engines having a dimensionally stable throat insert installed therein.
2. Discussion
In a regeneratively cooled bipropellant rocket motor the thruster is cooled using the latent heat of vaporization of the oxidizer flowing through coolant passages in the wall of the thruster chamber. In this type of cooled thruster design, there exists a large thermal gradient between the inner wall of the thruster chamber and the coolant passages. The thermal gradient is particularly large at the throat of the rocket thruster where heat loads and operating temperatures are at their greatest. These large thermal gradients cause local yielding of the material between the throat surface and the surface of the cooling passages. As a result of this yielding, there is a phenomena created called thermal ratcheting. Thermal ratcheting results in a radically inward shrinkage of the throat of the thruster after each firing or thermal cycle of the thruster. A reduction in throat area of up to forty-five percent has been observed within only sixty thermal cycles. Reductions in the throat area of this magnitude are clearly unacceptable after so few thermal cycles, particularly when considering that this type of thruster is often required to complete 700 cycles over the life of the thruster.
The problem of thermal ratcheting has proven to be difficult to overcome. Thermal gradients can be reduced by using materials having high thermal conductivity, but the yield strength and oxidation resistance of such materials is generally below that necessary for this type of application. Conversely, high strength materials which are more resistant to thermal ratcheting and exhibit good oxidation resistance typically have low thermal conductivities. The low thermal conductivity of such materials actually results in higher thermal gradients and related stresses, which may ultimately cause yielding and some thermal ratcheting.
While thermal ratcheting is not solely unique to bipropellant rocket thrusters, monopropellant thrusters are much less susceptible to thermal ratcheting because the thermal gradients are not as severe. However, any improvement in the reduction of thermal ratcheting which is developed for bipropellant rocket thrusters can of course be applied when the thrusters are operated as a monopropellant thruster, even though the need is not as great.